1. Field of the Invention
This invention relates to a rotary drill bit, and in particular to an improved rotary drill bit suitable for use in a steerable drilling system.
2. Description of the Related Art
It is known, when drilling well bores for use in hydrocarbon extraction, to use steerable drilling systems to provide control over the direction in which the well bore is being drilled, and hence over the path along which the well bore extends. One type of steerable drilling system uses a downhole motor to control the angular position of a housing relative to the formation being drilled, the housing including an angled drive shaft carrying a drill bit such that the axis of rotation of the drill bit is angled to the axis of rotation of the housing. By appropriate control over the speed at which the housing is rotated relative to the speed at which the downhole assembly as a whole is rotated, the angular position occupied by the housing, and hence the drilling direction can be controlled. A steerable drilling system of this type falls into a category of drilling systems known as point-the-bit systems. A number of other steerable drilling systems are known.
Where a point-the-bit drilling system type is used with a conventional drill bit, upon using the tool to cause a change in drilling direction the drill bit is tilted relative to the hole. Such tilting causes the bit to foul against the adjacent formation. In order for the desired amount of tilting of the bit to occur, the parts of the formation against which the bit fouls must be drilled even though drilling of those parts of the formation may not otherwise have been necessary. Obviously this is undesirable as it is uneconomic and may result in increased bit wear. It may also negatively impact upon the steering accuracy, and also cause an oversize borehole to be drilled. The present invention provides drill bits suitable for use in such applications in which changes in drilling direction can be achieved with reduced effort.